Stabilized isotactic polypropylene and method



3,022,267 Patented Feb. 20, 1962 3,022,267 STABILIZED ISO'FACTIC POLYPROPYLENE AND METHOD David- W. Young, Homewood; IlL, assi'gnor to Sinclair Refining Company, New York, N.Y., a corporation of This invention concerns a novel composition which comprises a major amount of isotactic polypropylene and a minor amount of a high molecular weight amide, in particular N-stearoyl p-amino phenol.

Isotactic polymers are head-tail-connected linear polymers, the monomer units of which ('CH =CHR) are distinguished by a simple, regular structure incapable of stereoisomerism. Their chains consist of sections which exhibit a uniform reiteration of monomer units with tertiary C atoms ofequal steric configuration. In isotactic polypropylene, the R group of the formula is a CH group. The structure of this polymer is illustrated inl97, Scientific American, number 3, pages 102 and 103 (September 1957).

The advantages of isotactic polymers have been cited by Natta in 68, Angewandte Chemie, 393403. (1956). These high-melting thermoplastic polymers, obtained from low-priced starting materials, yield fibers of' great tensile strength, which are useful astextile raw materials. Isotactic polypropylene threads have a tear strength up to more than 7 g./den. (.70 kg./ sq. mm.), comparableto the strengthof steel, but witha specific gravity of one-eighth as,- high as that of iron. Isotactic polypropylene surpasses polyethylene in many properties. This polypropylene has an averagemolecular weight. generally of from about 100,000 to 1,500,000Staudinger. The CH groups are located onthe same side of the carbon chain inv a helical structure, characterized by an identity period generally of from about 6.5 to 6.6 A. along the axis of the HardnessRockwell' ASTNI D -7 -5 1 R-sca'le 8 5-9 5. Melting point, F; 330-350. Specific heat-K ca1./gm., C. .0.4'6;

X-ray diffraction pattern of isotactic polypropylene in terms of Bragg spacings (d/n.) and intensities d./n. int. 631 A. 10

Natta, in 37, La Chimica and Industri'a (-Milan'), 8'8'-8- 900 (1955 attributes to the isotactic structure certainproperties which had previously not been found" in By drocarbons'. At ambient temperatures; these high-molecular-weight polymers are actually hard, crystalline solids. The low-molecular-weight isotactic polymers exhibitlike}- wise ahigh degree of crystallinity; For example, isotactic polypropylene specimens with a molecular weight of about 2,500 present the same crystalline polymeric structure as those polymers which have weights of several hundreds of thousands. The isotactic polymers are of great resistivity towards isomerization'eifects', which would modify the configuration of the branched methyl group, even in the presence of catalysts which act by an ionic) mechanism. Prolonged heating to 300450" C. in the absence of catalysts (or else to lower temperatures, but in the presence of various reagents which act by a freeradical mechanism), may cause a depolymeri'z'ation of the macromolecules, although not to a very large extent.

Fun-menace There are noteworthy differences in thedegrees of TABLE r l-order Soiubilityin Intrinsic Experim. transition 2 Initial a 1 Polymer viscosl spec. I temp, fusion v I p ccJg. gravity O. temp; Ether n-Hep- Toluene tame High-gnolecular-weight crystalline polypropyl- 2.40 0. 92' 158-160- -Insol' Insol-.. S011 0116. Low-n lolecular-weight crystalline polypropyl 0.17 0391 I49 Insulin. Sol Sol;

1 Amorphous polypropylene 0.55 0.85- 75 S01 Sol ,Highly sol.

Determinedat""0: using tetralin as asolveut;

isotactic. 3 Made atmosphere.

4 Atactic-mode according to'conventlonal procedures.

principal: chain. It has: ahigh melting point (e.g. 346

and is-"a solid'; highly crystalline substance with; for instance, the following physical properties:

ft. lb./in. 19.. Fusiontemperature, F. 31.6.4 to 3.38. Compressive strength ASTM D695 54 lb./in. 3 9,500-1.1,00.0. Dielectric constant (10 cycles/ sec.) 2,0---2.1.

by depolymerizing the hlgli-molecular-weight. crystallinematerialist-335? C. ln attetra'lin solution-ands nitrogen- A product which is insoluble in boiling n-heptane is practically completely crystalline. An ether-extracted polypropylene product. isco'mpletelly amorphous and has a much lower transistion temperature. After' sulfochlorination andvulcanization treatments, the amorphous'type' presents elastomer-like properties;

These isotactic polypropylenes can be prepared from. propylene with Ziegler type catalysts inclu'dingcomplexes consisting of'several substances; for instance, the chloride of a metal with an unfilled electron shell, and anorgano? metallic compoun'cl, underpolymerizationconditions including temperatures" above ambient temperatures; ezg; from about 68to 302 F. andpres'sures from about atmospheric or less up to about 30 atmo'splieresj o'r'mo're'; With catalysts ofjihigh selectivity; very'high yields of isotactic polymers maybe obtained. The" formation of 3 amorphous, non-crystalline polymers may be almost completely prevented.

Natta has found that the production of isotactic polymerizates of alpha-olefins requires the presence of solid catalysts which are chemically bound to the growing polymer chain. It is probable that such stereospecific catalytic processes are based on the chemisorption of the monomer-molecules on the surface of the solid catalyst, and that the adsorbed molecule presents itself always in a definite orientation on the growing chain.

This type of catalyst is characterized by the presence of compounds of a metal of the fourth to the eighth group of the transition elements in a lower stage of oxidation. The structure of their atoms is characterized by the incompleteness of the d-shell. Their compounds, in a lower stage of oxidation, can enter into associations with metal hydrides or with metal alkyls, whereby complexes are formed in which metal-hydrogen or metal-alkyl linkages are found in a highly polarized form.

In practical operation, the best catalysts are obtained from Ti, V, Cr and Zr. The polymerization catalysts are produced by reacting compounds of these metals with alkyls or hydrides of Na, Li, Ba or Al. Compounds of a lower stage of oxidation, such as VCl and TiCl enter into combinations with the metal hydrides or metal alkyls. This results in the formation of catalysts which act by an anionic mechanism. These catalysts are heterogeneous, due to the presence of a solid phase, which may be colloidal in dimension or microcrystalline.

Typical catalysts with an anionic mechanism-activity are those which are obtained by treating, with metal alkyls, solid crystalline lower halides of metals of the preferred transition element group. The quantity of metal alkyl required for accomplishing maximum catalytic activity, is relatively small: e.g. 0.1 mol. AlR per mol. of the halide of the transition element. Natta found it advantageous to operate with an excess of this metal alkyl compound, to exert a protective activity by destroying oxygen and moisture which may be present in trace amounts. Otherwise, these substances might poison the catalysts. Using the terminology developed for cationic catalysis, Natta hypothesizes that in this type of anionic catalysis the transition element compound may be regarded as the catalyst and the other compound, aluminum trialkyl or aluminum dialkylhydride, for instance, as the co-catalyst.

Natta'also found (69, Angewandte Chemie, 213-219 [1957]) that the final reaction velocity after an initial adjustment period is a linear function of the amount of the crystalline constituents of the catalyst and remains constant for a long period of time, sometimes more than thirty hours, when the concentration of the catalyst is 'sufiiciently low. The rate of reaction is also a linear function of the propylene partial pressure. The energy of activation of the over-all process lies at approximately 12,000-14,000 kcal./kmol., a value which is lower than that found in most of the radical-activated polymerization processes. The number of active centers created by the interaction between aluminum triethyl and titanium tnichloride surfaces does not vary markedly with time, if the catalyst has attained the final state.

As has been pointed out above, isotactic polypropylene made according to the process outlined by Natta has valuable properties for use as a synthetic film or fiber. However, it has been found that exposure of this material to light and air causes a darkening of such film and fiber products which seriously afiects their marketability. Since these polymers are essentially paraffinic in nature, and since the linearity of them is due to the absence of any active centers along the polymer chain, it is hypothesized that the polymer molecule is attacked at its end, since Natta theorizes that a vinylidene group appears at the end of the molecule when it breaks away from the growing surface of the catalyst. This invention, however, is not to be limited by this theory.

This invention consists in mixing with the isotactic polypropylene 0.005 percent to 5 percent of a high molecular weight amide which is soluble in the polymer, that is, N-stearoyl p-ann'no phenol to produce an oxygenstabilized product.

N-stearoyl p-amino phenol is sold under the trade name Suconox-IS. It is a white solid which melts at 131- 134 C. (268273 F.) and can be heated'to above 250 C. (482 F.) without any detectable evidence of decomposition, and appears to be non-toxic. In Suconox-l8 the stearoyl group is a mixture derived from commercial stearic acid and is, theoretically, primarily C117H35CO.

N-stearoyl p-amino phenol is prepared in general by reacting an approximately equimolar amount of stearic anhydride or dry stearic acid with the amino-phenol in the presence of a solvent or water-entraining agent such as benzene, toluene or xylene. The solid product is then dehydrated by evaporation and the product is obtained in relatively pure form.

The N-stearoyl p-amino phenol can be used in isotactic polypropylene over the concentration range of 0.005 percent to 5 percent. However, best results are obtained at about 0.01 to 0.25 percent by weight. It is important to obtain adequate dispersion of the N-stearoyl p-amino phenol in the polypropylene and this can be done by mixing the N-stearoyl p-amino phenol with the polymer isotactic polypropylene at a temperature about 5 to 10 F. above the crystalline'melting point of the polymer and holding this temperature for about 2 to 5 minutes, then removing the heat and letting the temperature drop to cool the mixture at the rate of about 15 to 30 F. in about 7 to 10 minutes. The polymer and anti-oxidant are agitated together as the system is cooled and the mixture reverts from a liquid to a solid.

When the isotactic polypropylene with 0.1 percent N- stearoyl p-arnino phenol is placed in a watch glass and held for 15 days in a circulating air oven in the dark at F. the product shows no change in color. An isotactic control with no anti-oxidant present gave a product in the same type test at the same time that was dark brown in color. The results are recorded in Table H.

TABLE II Isotactic polypropylene Original plus 0.1% Original N-stearoyl Property Original after 15 p-amino days at phenol 150 F. after 15 days at 150 F.

M.Pt., F 330 384 332 Mo]. weight 159, 000 47, 000 144, 000

Isotactic polypropylene has found uses as an additive in wax, white oil, soap and other compositions. This invention provides a means whereby the isotactic polypropylene may be stabilized when admixed with other materials as well as when it is used asthe major constituent of fiber, film or other materials, along with suitable amounts of other additives such as plasticizers, etc.

Although N-acyl p-amino phenols have been suggested in US. Patent No. 2,654,722, as a suitable stabilizer for various conventional polymers and elastomers of the more or less highly cross-linked type, these materials are apparently attacked by oxygen in a difierent manner from the attack by oxygen on isotactic polypropylene, since few materials which will stabilize cross-linked polymers have any stabilizing efiect on isotactic polypropylene, and since N-stearoyl p-amino phenol has a better stabilizing: effect on isotactic polypropylene than on non-isotactic polypropylene.

I claim:

1. A composition of matter consisting essentially of 5 solid, crystalline isotactic polypropylene and 0.005 to 5 percent of N-stearoyl p-amino phenol.

2. The composition of claim 1 Which contains 0.01 to 0.25 percent of N-stearoyl p amino phenol.

3. The composition of claim 2 resulting from mixing the ingredients at about 5 to 10 F. above the melting point of the polymer.

4. A method of stabilizing a solid, crystalline isotactic polypropylene composition which comprises mixing the polypropylene and 0.005 to 5 percent by weight of N- stearoyl p-amino phenol at a temperature about 5 to 10 F. above the crystalline melting point of the polypropylene and holding the mixture at this temperature for about 2 to 5 minutes.

5. The composition of claim 1 in which the isotactic polyropylene has an average molecular weight of from about 100,000 to 1,500,000 Staudinger.

6 6. The method of claim 4 which includes cooling the mixture at the rate of about 15 to 30 F. in about 7 to 10 minutes, after the holding step.

References Cited in the file of this patent UNITED STATES PATENTS 2,654,722 Young et a1. Oct. 6, 1953 2,705,227 Stamatoff Mar. 29, 1955 2,824,089 Peters et a1. Feb. 18, 1958 2,843,577 Friedlander et a1 July 15, 1958 2,908,670 Hagemeyer et a1 Oct. 13, 1959 2,921,048 Bell et al J an. 12, 1960 2,925,400 Tholstrup et a1 Feb. 16, 1960 2,939,860 Schramm June 7, 1960 2,957,849 Kennedy Oct. 25, 1960 

1. A COMPOSITION OF MATTER CONSISTING ESSENTIALLY OF SOLID, CRYSTALLINE ISOTACTIC POLYPROPYLENE AND 0.005 TO 5 PERCENT OF N-STEAROYL P-AMINO PHENOL. 